Watt-hour meters or electrical energy meters are widely used in almost every household to determine the amount of energy delivered by the electric utility. Regardless of the specific principle used, they all measure and register the integral over time of the power in the circuit. Generally, the watt-hour meter converts the power into a mechanical or an electrical signal and a counter integrates and displays the value of the total energy that has passed through the watt-hour meter. Watt-hour meters may use electronic or mechanical components.
An electric motor is a rather mechanical approach for an energy meter. The torque of this electric motor corresponds to the power consumed in a household or any electronic circuit to which it is connected. A Ferraris or induction-type meter is used for AC energy measurement. Ferraris meters widely used and measure the energy consumption of the vast majority of domestic and industrial users of electric power throughout the world. However, Ferraris energy meters are rather bulky and noisy. They are being slowly replaced by electronic energy meters that can offer additional features like automatic meter reading (AMR).
Electronic solutions preferably rely on shunt resistors coupled directly into the electric line to be measured. The voltage drop across the shunt device corresponds to the current through the electrical line. The voltage level of the electrical line is predetermined. An analog to digital converter digitizes the analog voltages corresponding to the current through the electrical line and to the voltage level at the electrical line. The product of the digital values of voltage and current is the instantaneous power consumed through the electrical line. The integral of this instantaneous power is the energy consumed. A current transformer can be used to measure the current instead of a shunt. Other solutions rely on Rogowski coils or Hall-sensors. However, the cheapest solution uses a shunt.
A major problem to be solved by multi-phase watt-hour meters is the electrical isolation between phases. The electronic meters are coupled through shunt devices and voltage dividers to the electrical line to be measured. These electronic meters experience the high electrical potentials of the electrical lines of up to several hundreds volts. Analog-to-digital converters (ADCs) used in these electronic meters are not designed to withstand such high voltages. At least a part of the electronic components coupled to the electrical line must be electrically decoupled or isolated so that they can float and take the potential of the electrical line.
FIG. 1 illustrates a typical prior art solution to achieve electrical isolation. Three electrical lines correspond to the three phases P1, P2 and P3 of a domestic or industrial power supply network. Three energy meter front ends EMFE1, EMFE2 and EMFE3 are coupled to the phases P1, P2, P3. Electrical front ends EMFE1, EMFE2 and EMFE3 measure the currents and voltages on respective phases. The energy meter front ends EMFE1 to EMFE3 include analog-to-digital converters which convert the analog input voltages representing the currents and voltages through the electrical lines into digital values. The digital values are transmitted to control block CONTROL which may be a microcontroller. Control block CONTROL determines the power in each phase P1, P2 and P3 and the total power or energy consumed. The result is displayed on a liquid crystal display LCD. Three optocouplers OC1 to OC3 establish electrical isolation between the control stage CONTROL and the three energy meter front ends EMFE1 to EMFE3. Directly coupling the three energy meters to control stage CONTROL is impossible because the instantaneous potentials (voltage levels) at the three phases P1 to P3 can differ by hundreds of volts dependent on the specific regional or national standards. Thus each energy meter front end must be electrically isolated by an individual optocoupler. However, optocouplers are rather expensive and they consume a substantial amount of energy themselves which is undesired. If more information or data is to be transmitted even more optocouplers are needed.
FIG. 2 shows an alternative implementation of a prior art energy meter. In FIG. 2 one of the phases P1 is directly coupled to an energy meter front end EMFE provided in the control stage CONTROL at an electrically isolated input to control front end EMFE. This reduces the number of optocouplers required. This alternative is not sufficient to reduce size and costs of the solution to an acceptable amount.